The research efforts of the Center for Population Research have been on projects relating to the molecular and cellular biology of hormones, physiology of reproduction, endocrinology of parturition, cytogenetics of early embryos, antifertility agents and contraceptive developments. A brief summary of the progress in the field of biology of hormones and physiology of reproduction is presented: Biology of Hormones: Avidin mRNA from progesterone-treated chicks' oviducts has been purified. It was established that the purified mRNA contained bound proteins. By interacting labelled hCG with rat testis or ovaries, evidence was obtained to suggest that the hormone underwent modification during the interaction. Physiology of Reproduction: The sperm count in the epididymis and the serum testosterone levels of male rats increased between days 43 and 46. The administration of antisera to bovine LH depressed the testosterone level, sperm count and ventral prostate weight. Androgens added to rat epididymis explants stimulated the incorporation of amino acid into proteins and uridine into RNA. Blood flow through the testis and countercurrent exchange between the internal spermatic vein and artery were determined by using 85Krypton and 133Xenon blood clearance techniques. Prostaglandin F2 alpha caused a markedly prolonged decrease in testis blood flow. Preliminary results indicate that countercurrent exchange of testosterone between spermatic veins and artery does take place. Endometrial blood flow in the Rhesus monkey throughout the menstrual cycle was established by measuring 133 Xenon clearance. Intrauterine instillation of prostaglandin F2 alpha effected a decrease in the endometrial blood flow and plasma progesterone level. Vasectomy in rats and monkeys did not alter the mating behavior, testosterone secretion rates, blood flow through the testis and histology of the testis and accessory sex organs. Rhesus monkeys were immunized with an estrone-BSA conjugate. Plasma estrogen levels were elevated in proportion to the increase in antibody titers. Some immunized monkeys became anovulatory which was correlated with low plasma progesterone levels. Intra-ovarian pressure was measured by using a miniature latex balloon connected to a thin polyethylene catheter to permit continuous recording.